Recently, as world-wide environmental problems and safety to a human body arouse concern, it has been increasingly required that electric/electronic products be less harmful and more safe, in addition to flame retardancy requirement. More specifically, the electric/electronic products are needed hardly to burn and scarcely to generate a harmful gas and fume. Conventional printed wiring boards, on which electric/electronic products are to be mounted, usually have a substrate formed of glass-epoxy. Generally, a brominated epoxy resin, in particular, a tetrabromobisphenol A type epoxy resin, containing bromine which serves as a flame-retardant, is used as the epoxy resin in the substrate.
Such a brominated epoxy resin has good flame retardance. However, it generates a harmful hydrogen halide (hydrogen bromide) gas when burned. For this reason, the use of the brominated epoxy resin is being suppressed. In the circumstances, compositions have been developed, in which a non-halogen flame-retardant such as a nitrogen compound, phosphorus compound or an inorganic compound is added to the conventional epoxy resin (see, for example, British Patent No. 1,112,139 and Jpn. Pat. Appln. KOKAI Publication No. 2-269730). However, these additives imparting flame retardance have problems: they impose undesirable effects on curing of the epoxy resin, decrease the moisture vapor resistance of the cured composition, and impair impregnation of the composition into a glass cloth.
Another feature which is important equally to the flame retardance, and which the printed wiring board should have in view of safety improvement, is tracking resistance. The tracking resistance is particularly required for a substrate having a high voltage circuit, such as a printed wiring board for use in air conditioners and televisions.
To impart the tracking resistance, a chain hydrocarbon, a cyclic compound or an inorganic material has been added to a resin composition for use in the printed wiring board, as a substance which is hardly carbonized. However, these substances are disadvantageous in that the chain hydrocarbon and cyclic compound reduce heat resistance of the printed wiring board, and the inorganic material must be used in a large amount.
Accordingly, an object of the present invention is to provide an epoxy resin composition which exhibits good flame retardance, without containing and also good tracking resistance.
Another object of the present invention is to provide a prepreg impregnated with such an epoxy resin, as well as a laminate, copper-clad laminate and printed wiring board which are formed by using the prepregs.